1. Field of the Invention
The present invention relates to radar, and more particularly, to a system and method for determining the presence of multipath conditions and determining the position of and the tracking of targets in the presence of multipath conditions.
2. Description of Prior Art
When detecting and tracking an object or target by radar, there are times when target reflected signals travel two different paths between antenna and target.
One path will be a straight line and the other path will involve the reflection of target signals off the ground or water at shallow grazing angles. The radar antenna may receive both signals in the main receiving beam of the antenna. When a radar is receiving signals under multipath conditions, the radar will track an apparent target signal at an elevation position determined by the composite signal of the target and image signals.
Heretofore, several methods and systems have been devised or proposed to eliminate the problem of tracking targets in a multipath environment. For example in U.S. Pat. No. 3,097,356 issued on July 9, 1963, a system is shown wherein the width of the radar beam is narrowed to produce an area responsive to targets of only 0.2 degrees angular width. However, within this target area, the multipath problems persist. In addition, the tracking lag, where the actual antenna position lags behind the target position due to inertia and other delays in the radar control system, must be considerably reduced in order to track the target.
U.S. Pat. No. 3,197,776 issued on July 27, 1965 minimizes the effect of echoes received from the ground by using a monopulse radar with typical monopulse target annular error detection capability disposed in the vertical plane. The echoes received from below the center of the antenna beam will be received by the monopulse antenna producing one polarity of signal, while the echoes received from above the center of the antenna beam will produce an opposite polarity signal. The radar described in U.S. Pat. No. 3,197,776 is responsive only to the polarity of the signal corresponding to echoes received above the center axis of the antenna beam; thus, the effect of ground reflections is reduced when the center axis of the beam of the radar is above ground (horizon). However, because the center axis of the beam is above ground, a substantial area between the earth and the center axis of the beam may be rejected allowing a target in that area to go undetected.
A method of target tracking wherein the track signal is inhibited during periods of multipath conditions is described in U.S. Pat. No. 3,130,402 issued on Apr. 21, 1964. Two channels are employed to give independent target position information. As long as both channels provide the same target position information, the target is tracked; whenever the position information differs, as it would be for target returns due to multipath conditions, a track signal gate is inhibited and tracking stops. This radar relies on the assumption that the period of inhibited tracking is small with respect to the total tracking period, and that two equal, but erroneous position signals will not be produced. Continuous or severe image or false target signals produced from ground or water reflections may cause substantial periods of time where tracking is inhibited.
A prior art method of detecting multipath conditions operates on the knowledge that a direct radar reflection (from a target to the radar antenna) has a different path than a multipath reflection, from the target to the reflecting surfaces to the radar antenna, and that these different reflection paths can frequently be converted to signals having energy in quadrature .+-.90.degree. out of phase relative to each other, so that when the quadrature signals are received, multipath conditions are assumed. However, not all multipath conditions produce quadrature signals, therefore the elevation measurements produced by multipath reflections not having quadrature signals appear to be true measurements in free space conditions.
It is desirable to provide a radar system and method for determining whether or not a multipath condition exists.
It is desirable to provide a radar system and method for tracking a target through the transition from free space to a multipath condition with minimum tracking disturbance.
It is desirable to provide a radar system and method for reliably tracking targets with minimized track error under multipath conditions.
It is desirable to provide a radar system and method for determining the position of a target under multipath conditions.